When producing electric-resistance-welded pipe, first the two ends of a strip-shaped steel plate are forced together by using a plurality of rolls to thereby shape the strip-shaped steel plate into a tube. Further, an induction coil or electrode contact tips arranged substantially coaxially with the tube shaped strip-shaped steel plate are used to run a high frequency current to the two ends of the tube shaped strip-shaped steel plate (weld faces). The Joule's heat generated at the two ends of the strip-shaped steel plate due to this high frequency current heats the weld faces to melt. Further, the heated and melted parts are pressed by squeeze rolls to join them.
When producing electric-resistance-welded pipe in this way, the weld faces start to melt from the edge parts. Therefore, the molten state at the center part of the weld faces becomes a problem. That is, if the center part of the weld faces does not melt, even if the squeeze rolls apply pressure, the weld faces will not be joined or the strength of the electric-resistance-welded pipe produced will be insufficient. On the other hand, if the center part of the weld faces overly melts, scale (oxides) which had been pushed out from the weld faces will be pulled into the weld faces by electromagnetic force. This scale will cause defects.
Therefore, it is extremely important to accurately monitor the welding state of electric-resistance-welded pipe in an electric-resistance-welded pipe production line.
Therefore, Japanese Patent Publication (A) No. 5-123874 discloses to use a camera placed directly above a weld point of electric-resistance-welded pipe being produced so as to take a still image of the surroundings of the weld point, convert the still image to a digital image, and perform image processing to find the position of the weld point.
Further, Japanese Patent Publication (A) No. 62-203680 discloses to run a fiber scope through the inside of electric-resistance-welded pipe being produced and process the image of the welded parts obtained by this fiber scope to estimate the butting state of the electric-resistance-welded pipe.